This sort of thing rarely happens, if ever, as per Cavalli-Sforza. The rural agricultural populations that are the base of all civilized cultures are very hard to eradicate. Conquerors generally just replace the upper classes and at best the urban centers, which historically have been population sinks - where races go to die.
Also, other factors such as historically linked languages - Coptic is related to Arabic, etc., suggest that the present Egyptian population is in fact descended mainly from the ancient Egyptian population.
Yes, but Arabic isn’t a Celtic language. That’s the problem here.